Deep groove ball bearings comprise an inner ring with a ball groove (inner raceway), an outer ring with a ball groove (outer raceway) and a plurality of balls in the grooves that are circumferentially separated by a cage that loosely engages the balls. Grease is commonly employed to lubricate the balls and raceways to lower friction and prevent direct metal to metal contact. The grease typically consists of a metal soap or other thickening agent that holds an oil or synthetic fluid and some additives to improve the lubricating or other properties. Frequently, a shield is attached to the outer ring to help contain the grease and exclude dirt from the raceways.
High speed ball bearing operation takes place when the dn number for the ball bearing exceeds about 150,000 mm/min where dn=shaft dia in mm.times.rpm. For a 25 mm shaft, a speed greater than 6,000 rpm would be considered high speed operation. It is known that the presence of excess grease in the raceway of the bearing causes heating of the bearing and of the grease, and may ultimately lead to degradation of the grease and a subsequent reduction of bearing life, as discussed, for example, by Simmons et al. in U.S. Pat. No. 4,082,381. Shortening of the bearing life is a significant problem when the bearing must be changed frequently to avoid or respond to unscheduled outages. In some applications, the heat produced in the bearing causes heating of elements in a machine that may be detrimental to a process or a product utilizing such machine. Some bearing manufacturers have suggested that the way to handle the high speed is to use a "hybrid ceramic" bearing having ceramic balls and metal rings, but this is more expensive and does not eliminate heating of the grease. Another solution has been to use an oil mist lubrication system, but this is more expensive to provide and maintain, and the mist is sometimes difficult to contain and can produce environmental problems for people and products.
Bearing and grease manufacturers have recognized the problems this excess grease causes for high speed operation and at least one grease manufacturer has recommended special run-in procedures to work the grease out of the ball path for high speed operation. These procedures are time-consuming and cumbersome to practice and expensive to implement, especially when large numbers of bearings are involved.
An object of my invention is to provide a better solution to these problems. My invention also provides advantages for bearing assemblies in other than high speed operations, namely improved longevity.
Ball bearings have been operated in the past at speeds of 12,000 rpm by using a conventional bearing provided by NTN Bearing Corporation of America and removing one of the shields to permit air circulation through the bearing to lower the operating temperature. I have found that this has kept the peak machine element temperature adjacent the bearing below 110.degree. F. (43.degree. C.). This has worked, but I have found that this has allowed grease to be slung out of the bearing so the bearing life has been only about 6 months. In another test, the grease was cleaned out of a conventional bearing and only a drop of grease was placed in the raceway, only a bead of grease was placed on the inner ring land, and the outer ring shields were reinstalled which pushed some grease up against the bearing cage. This resulted in a peak machine element temperature at startup slightly greater than 110.degree. F. (43.degree. C.), and a temperature slightly below 110.degree. F. (43.degree. C.) after 24 hours. Such an initial peak temperature is undesirably high.
Conventional bearings are designed for inner ring rotation and bearing shields are therefore mounted on the normally stationary outer ring. The shields help exclude dirt from the bearing and contain the grease that is normally packed in the bearing to fill 30-50% of the free space within the bearing between the shields on each side of the bearing. In some cases it has been considered desirable to mount the conventional bearing for outer ring rotation. The shield mounted on the outer ring would then rotate with the outer ring of the bearing. I believe this is undesirable for high speed operation since it is my opinion that the rotating shields further agitate and heat the grease, and facilitate leakage of the grease past the shields. I believe that anything more than a thin film of grease causes undesirable heating, but I also believe that providing only a thin film and nothing more does not provide a sufficient reserve of grease for long term bearing operation. The oils in the grease are eventually wicked to the bearing which eventually throws the oil off or causes vaporization, oxidation, and loss of the oils so only the base thickener in the grease remains.
Hall, U.S. Pat. No. 4,941,757, disclosed a flexible, arcuate grease reservoir member 32 of generally C-shaped configuration, attached to the stationary outer ring of a bearing and holding a reservoir of grease, with spaced ends 34 and 36, a generally cylindrical outer surface 38 and a generally cylindrical inner surface 42 (col 2, lines 55 et seq.). The surfaces 38 and 42 of Hall's C-shaped member 32 extended about 240.degree. to 260.degree. about their axis so the spaced ends 34 and 36 could be flexed during mounting of the reservoir member on the bearing and removal from the bearing (col 3, lines 14 et seq.). In other words, Hall's reservoir member 32 did not extend all the way round the bearing. Hall's reservoir 10 replaced one of the conventional shields (col 2, lines 52-54). Hall's reservoir 10 was for use with a conventional bearing (col 2, lines 30-32), i.e., one packed with grease (col 1, lines 7 et seq.). His grease reservoir was designed to be easily attached and removed from the bearing without removing the bearing from its mounting or even having to remove the shaft passing through the bearing (col 1, lines 30-35 and 48-60). Such a packed bearing would cause the grease to heat if it were to be used in high speed operations, and Hall's replacement of a shield by his C-shaped reservoir member 32 would permit grease to be ejected by centrifugal forces during high speed operation.